A known image reading apparatus includes a reading unit and a conveying device. The conveying device conveys a sheet to the reading unit selectively along a main path and a bypass path. The main path and the bypass path are joined, at a junction, into a common downstream path portion shared by the main path and the bypass path. A sheet detection sensor is disposed at a sheet support which supports a sheet inserted into the bypass path.
The reading unit is disposed in the common downstream path portion to read an image of the sheet conveyed by the conveying device. The known image reading apparatus generates shading correction data by means of the reading unit which reads a color reference member. Data of an image read by the reading unit is corrected using the shading correction data to improve image data quality.